Sanitary paper tissue products are widely used. Such items are commercially offered in formats tailored for a variety of uses such as facial tissues, toilet tissues and absorbent towels. The formats, i.e. basis weight, thickness, strength, sheet size, dispensing medium, etc. of these products often differ widely, but they are linked by the common process by which they originate, the so-called creped papermaking process.
Creping is a means of mechanically compacting paper in the machine direction. The result is an increase in basis weight (mass per unit area) as well as dramatic changes in many physical properties, particularly when measured in the machine direction. Creping is generally accomplished with a flexible blade, a so-called doctor blade, against a Yankee dryer in an on machine operation.
A Yankee dryer is a large diameter, generally 8-20 foot drum which is designed to be pressurized with steam to provide a hot surface for completing the drying of papermaking webs at the end of the papermaking process. The paper web which is first formed on a foraminous forming carrier, such as a Fourdrinier wire, where it is freed of the copious water needed to disperse the fibrous slurry is generally transferred to a felt or fabric in a so-called press section where de-watering is continued either by mechanically compacting the paper or by some other de-watering method such as through-drying with hot air, before finally being transferred in the semi-dry condition to the surface of the Yankee for the drying to be completed.
The various creped tissue paper products are further linked by common consumer demand for a generally conflicting set of physical properties: A pleasing tactile impression, i.e. softness while, at the same time having a high strength and a resistance to linting and dusting.
Softness is the tactile sensation perceived by the consumer as he/she holds a particular product, rubs it across his/her skin, or crumples it within his/her hand. This tactile sensation is provided by a combination of several physical properties. One of the most important physical properties related to softness is generally considered by those skilled in the art to be the stiffness of the paper web from which the product is made. Stiffness, in turn, is usually considered to be directly dependent on the strength of the web.
Strength is the ability of the product, and its constituent webs, to maintain physical integrity and to resist tearing, bursting, and shredding under use conditions.
Linting and dusting refers to the tendency of a web to release unbound or loosely bound fibers or particulate fillers during handling or use.
Creped tissue papers are generally comprised essentially of papermaking fibers. Small amounts of chemical functional agents such as wet strength or dry strength binders, retention aids, surfactants, size, chemical softeners, crepe facilitating compositions are frequently included but these are typically only used in minor amounts. The papermaking fibers most frequently used in creped tissue papers are virgin chemical wood pulps.
As the world's supply of natural resources comes under increasing economic and environmental scrutiny, pressure is mounting to reduce consumption of forest products such as virgin chemical wood pulps in products such as sanitary tissues. One way to extend a given supply of wood pulp without sacrificing product mass is to replace virgin chemical pulp fibers with high yield fibers such as mechanical or chemi-mechanical pulps or to use fibers which have been recycled. Unfortunately, comparatively severe deterioration in performance usually accompanies such changes. Such fibers are prone to have a high coarseness and this contributes to the loss of the velvety feel which is imparted by prime fibers selected because of their fiaccidness. In the case of the mechanical or chemi-mechanical liberated fiber, high coarseness is due to the retention of the non-cellulosic components of the original wood substance, such components including lignin and so-called hemicelluloses. This makes each fiber weigh more without increasing its length. Recycled paper can also tend to have a high mechanical pulp content, but, even when all due care is exercised in selecting the wastepaper grade to minimize this, a high coarseness still often occurs. This is thought to be due to the impure mixture of fiber morphologies which naturally occurs when paper from many sources is blended to make a recycled pulp. For example, a certain wastepaper might be selected because it is primarily North American hardwood in nature; however, one will often find extensive contamination from coarser softwood fibers, even of the most deleterious species such as variations of Southern U.S. pine. U.S. Pat. No. 4,300,981, Carstens, issued Nov. 17, 1981, and incorporated herein by reference, explains the textural and surface qualities which are imparted by prime fibers. U.S. Pat. No. 5,228,954, Vinson, issued Jul. 20, 1993, and U.S. Pat. 5,405,499, Vinson, to issue Apr. 11, 1995, both incorporated herein by reference, disclose methods for upgrading such fiber sources so that they have less deleterious effects, but still the level of replacement is limited and the new fiber sources themselves are in limited supply and this often limits their use.
Applicants have discovered that another method of limiting the use of wood pulp in sanitary tissue paper is to replace part of it with a lower cost, readily available filling material such as kaolin clay or calcium carbonate. While those skilled in the art will recognize that this practice has been common in some parts of the paper industry for many years, they will also appreciate that extending this approach to sanitary tissue products has involved particular difficulties which have prevented it from being practiced up to now.
One major restriction is the retention of the filling agent during the papermaking process. Among paper products, sanitary tissues are at an extreme of low basis weight. The basis weight of a tissue web as it is wound on a reel from a Yankee machine is typically only about 15 g/m.sup.2 and because of the crepe, or foreshortening, introduced at the creping blade, the dry fiber basis weight in the forming, press, and drying sections of the machine is actually lower than the finished dry basis weight by from about 10% to about 20%. To compound the difficulties in retention caused by the low basis weight, tissue webs occupy an extreme of low density, often having an apparent density as wound on the reel of only about 0.1 g/cm.sup.3 or less. While it is recognized that some of this loft is introduced at the creping blade, those skilled in the art will recognize that tissue webs are generally formed from relatively free stock which means that the fibers of which they are comprised are not rendered flaccid from beating. Tissue machines are required to operate at very high speeds to be practical; thus free stock is needed to prevent excessive forming pressures and drying load. The relatively stiff fibers comprising the free stock retain their ability to prop open the embryonic web as it is forming. Those skilled in the art will at once recognize that such light weight, low density structures do not afford any significant opportunity to filter fine particulates as the web is forming. Filler particles not substantively affixed to fiber surfaces will be torn away by the torrent of the high speed approach flow systems, hurled into the liquid phase, and driven through the embryonic web into the water drained from the forming web. Only with repeated recycling of the water used to form the web does the concentration of particulate build to a point where the filler begins to exit with the paper. Such concentrations of solids in water effluent are impractical.
A second major limitation is the general failure of particulate fillers to naturally bond to papermaking fibers in the fashion that papermaking fibers tend to bond to each other as the formed web is dried. This reduces the strength of the product. Filler inclusion causes a reduction in strength, which if left uncorrected, severely limits products which are already quite weak. Steps required to restore strength such as increased fiber beating or the use of chemical strengthening agents is often restricted as well.
The deleterious effects of filler on sheet integrity also often cause hygiene problems by plugging press felts or by transferring poorly from the press section to the Yankee dryer.
Finally, tissue products containing fillers are prone to lint or dust. This is not only because the fillers themselves can be poorly trapped within the web, but also because they have the aforementioned bond inhibiting effect which causes a localized weakening of fiber anchoring into the structure. This tendency can cause operational difficulties in the creped papermaking processes and in subsequent converting operations, because of excessive dust created when the paper is handled. Another consideration is that the users of the sanitary tissue products made from the filled tissue demand that they be relatively free of lint and dust.
Consequently, the use of fillers in papers made on Yankee machines has been severely limited. U.S. Pat. No. 2,216,143, issued to Thiele on Oct. 1, 1940, and incorporated herein by reference discusses the limitations of fillers on Yankee machines and discloses a method of incorporation which overcomes those limitations. Unfortunately, the method requires a cumbersome unit operation to coat a layer of adhesively bound particles onto the felt side of the sheet while it is in contact with the Yankee dryer. This operation is not practical for modern high speed Yankee machines and, those skilled in the art will recognize that the Thiele method would produce a coated rather than filled tissue product. A "filled tissue paper" is distinguished from "coated tissue paper" essentially by the methods practiced to produce them, i.e. a "filled tissue paper" is one which has the particulate matter added to the fibers prior to their assembly into a web while a "coated tissue paper" is one which has the particulate matter added after the web has been essentially assembled. As a result of this difference, a filled tissue paper product can be described as a relatively lightweight, low density creped tissue paper made on a Yankee machine which contains a filler dispersed throughout the thickness of at least one layer of a multi-layer tissue paper, or throughout the entire thickness of a single-layered tissue paper. The term "dispersed throughout" means that essentially all portions of a particular layer of a filled tissue product contain filler particles, but, it specifically does not imply that such dispersion necessarily be uniform in that layer. In fact, certain advantages can be anticipated by achieving a difference in filler concentration as a function of thickness in a filled layer of tissue.
Therefore, it is the object of the present invention to provide for a tissue paper comprising a fine particulate filler which overcomes the aforementioned limitations of the prior art. The tissue paper of the present invention is soft, contains a retentive filler, has a high level of tensile strength, and is low in dust.
This and other objects are obtained using the present invention as will be taught in the following disclosure.